Male howler monkeys with deep voices are definitely compensating for something. A large vocal tract may be a sign of small testes, researchers report October 22 in Current Biology.

Known for their grandiose bellows, male howlers use their vocalizations to woo females and scare off other suitors. To determine how roars relate to other reproductive traits, researchers examined the hyoid bone, which expands the vocal tract and amplifies calls.

Males with bigger hyoids produce deeper roars, an acoustic analysis confirmed, creating the illusion of a larger animal. The researchers also found that in howler species where there’s just one male per group, males tend to have larger hyoids and smaller testes; males that reside in groups with multiple males tend to have smaller bones but bigger testes.

For howlers, the researchers say, there’s an evolutionary trade-off between seductive vocals, a trait that helps them get girls, and sperm production, a trait that helps them get girls pregnant.